Flexible heat exchanger

ABSTRACT

The present invention relates to a heat exchanger which comprises a flexible sheet consisting of a porous material, such as polyvinyl alcohol, sandwiched between two liquid impermeable layers, the liquid impermeable layers defining between them and between fluid-tight edges, a liquid transfer space within said sheet and at least one liquid inlet and at least one liquid outlet for respective connection to a source and a drain of a heat transfer liquid, with a flow path defined between the liquid inlet and liquid outlet within said liquid transfer space.  
     According to one embodiment, the heat exchanger is comprised of an internal PVA sheet sandwiched between two layers of adherent, film forming material. According to one preferred embodiment, the film forming material is silicon.  
     The heat exchanger of the present invention is preferably formed into a garment for overlaying a substantial portion of a subject&#39;s body, for controlling its body temperature.

FIELD OF THE INVENTION

[0001] The present invention relates to a heat exchanger.

BACKGROUND OF THE INVENTION AND PRIOR ART

[0002] Most heat exchangers hitherto known are used for controlling the temperature of a variety of elements found in manufacturing industry, the process industry, buildings, vehicles, agriculture, medical equipment etc. Heat exchangers are found in many different configurations, however, most are characterized by rigid constructions, typically metal made and having simple geometric shapes.

[0003] One application of heat exchangers is during medical procedures, where a subject's body temperature has to be controlled. For example, during open heart surgery, the body temperature has to be controllably reduced to about 35° C. and then at the end of the operation increased back to normal body temperature. Furthermore, under general anesthesia, the natural physiological mechanisms which operate to maintain a body temperature may fail, and the problem becomes particularly acute in cases where exposed internal body organs come into contact with the ambient air which results in excessive heat loss. This problem is further aggravated by the fact that operating rooms are very often heavily cooled. Following surgical procedure, it may take some time until the body acquires back its ability to control its temperature.

[0004] Similarly to human subjects, the problem of controlling body temperature is also experienced during surgical procedures performed on animals.

[0005] Overall, controlling the body temperature is one of the serious problems facing the medical staff when performing operations.

[0006] U.S. Pat. No. 4,844,072, describes a system which may be used in thermal therapy and which includes a thermal pad with internal channels for carrying a temperature-control liquid therethrough. A thermal bandage which achieves a similar purpose is described in U.S. Pat. No. 4,962,761 through which heat control liquid is circulated when placed in contact with the body. In accordance with this patent, several such matters may be placed in series and fitted into a planar array or into a garment.

[0007] U.S. Pat. No. 5,269,369 discloses a temperature regulation system for human body which makes use of heat pipes which are incorporated into a garment, which distributes energy to and from portions of the human body.

[0008] A tube laminated heat transfer article for placing against a human body for transfer heat to or from the human body is disclosed in U.S. Pat. No. 5,755,275. A heating and/or cooling pad having the shape of the human body which is placed beneath the patient to allow heat or cold to radiate upwards is disclosed in U.S. Pat. No. 5,785,716.

[0009] MicroClimate Systems Inc., Stanford, Mich., U.S.A., market a series of portable personal cooling systems (sold under the trademarks KOOLVEST, KOOLJACKET, KOOLBAND, KOOLPAID and others) which are intended for use by healthy persons who perform a physical activity in a hot environment. The system consists of a garment (it may be a vest, jacket, ski cap, a cervical collar, etc.) with tubing embedded therein in which water flows propelled by a battery-powered pump. Water passes through ice or through a cooling device before entering the tubing within the garment, The user can control the pump rate, the temperature, etc. A similar product line is also provided by Mallinckrodt Inc., St. Louis, Mo., U.S.A. and others.

[0010] A system and method for heat control of a living body which may make use of a heat exchanger garment is disclosed in PCT Application WO 99/44552.

SUMMARY OF THE INVENTION

[0011] As may be known by those versed in the art, liquid-based heat exchangers are preferable over air heat exchangers. When designing a heat exchanger for body temperature control there are numerous factors which need to be considered. An ideal heat exchanger will require, inter aha, good heat transfer and low pressure drop, it should be flexible and stretchable (i.e. elastic), in order to conform with the body's shape, provide good contact therewith, reduces the number of sizes needed to be kept in stock and facilitates access to treated sites when used during medical procedures; should be resistant to blockage by inadvertent folding; should have good thermal insulation from the surrounding area; good liquid distribution and it should be cost effective. As may be understood by those versed in the art, when used for controlling the temperature of a living body, the heat exchanger should typically also be lightweight (e.g. by reducing its liquid content or by constructing it from a lightweight material), breathable, soft (e.g. to minimize pressure sores) and have an inner absorbent surface (e.g. to absorb body secretions). In the particular area of disposable body beat exchangers for medical use the design must facilitate disposal in an environmentally acceptable way in view of the large number of units involved.

[0012] The present invention aims to provide a novel liquid heat exchanger having, to a substantial extent the above detailed characteristics.

[0013] Thus, the present invention provides a liquid heat exchanger comprising:

[0014] a flexible sheet comprising a porous material sandwiched between two liquid impermeable layers; the liquid impermeable layers defining between them and fluid-tight edges, a liquid transfer space within said sheet; and

[0015] at least one liquid inlet and at least one liquid outlet for respective connection to a source and a drain of a heat transfer liquid, with a flow path defined between the liquid inlet and liquid outlet within said liquid transfer space.

[0016] This new design of a heat exchanger has certain new features which give rise to new potential uses in the different areas hitherto known to make use of heat exchangers. These features include, inter alia, flexibility and elasticity, hence the ability to conform to irregular shapes; low cost, therefore disposable, high corrosion resistance and capable of being produced in large area formats and in any shape.

[0017] In accordance with one preferred embodiment of the invention, a heat exchanger is formed from a sheet of porous material (hereinafter “porous sheet”), typically about 1-3 mm in thickness, the external faces of which are sintered or otherwise treated to close the surface pores to form a liquid impermeable layer, or overlaid by a liquid impermeable film. In accordance with one embodiment, closing of surface pores to form a liquid impermeable layer may be achieved by applying an adhesive, typically in a liquid or semi-liquid state, onto the faces of the porous sheet of the kind which after curing forms a liquid impermeable film. In accordance with another embodiment, the surface pores are closed by sintering, e.g. heat-induced sintering, whereby an integral liquid impermeable layer is formed.

[0018] The impermeable layer should be formed such that it will not impair the porous sheet's flexibility and elasticity, among other of its features.

[0019] In accordance with another embodiment of the invention the heat exchanger is constructed from at least two porous sheets, attached to one another, with a liquid impermeable interface between them. The liquid transfer space is, in accordance with this embodiment, formed in one of the porous sheets between the liquid impermeable interface and a liquid impermeable layer on the other face of this sheet. The other porous sheet may be on the inward, body-facing side of the liquid transfer space, in which case it may serve as an absorbent of body fluids; or may be on the outward side of the liquid transfer space, in which case it may serve for thermal insulation.

[0020] The porosity of the liquid transfer space causes the liquid to assume a tortuous path within the pores of the porous sheet, which causes the liquid to essentially evenly distribute within the liquid transfer space. However, in accordance with one preferred embodiment, in order to ensure a more homogenous flow of liquid throughout the space, several liquid inlets may be provided at different locations to define together a moving liquid front (such a front consisting of liquid exiting from the openings at about the same time). In accordance with another preferred embodiment, said at least one liquid inlet and said at least one liquid outlet comprise a respective feed and drain manifold. The feed and drain manifolds are another measure to improve distribution of the liquid within the liquid transfer space.

[0021] Typically, the manifolds comprise a plurality of openings in a liquid duct, communicating with said liquid transfer space. Said duct, in accordance with a preferred embodiment, is integrally formed with said flexible sheet (i.e. is made of the same or substantially the same material as the flexible sheet).

[0022] The heat exchanger of the invention is preferably used for controlling a subject's body temperature, such as new borns as well as individuals, particularly undergoing medical procedure.

[0023] Within this aspect of the invention, there is also provided a garment for controlling a subjects body temperature, comprising a heat exchanger of the invention.

[0024] The invention will now be further described by way of example only to the following non-limiting specific embodiments with occasional reference to the annexed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 shows an embodiment of a heat exchanger in accordance with an embodiment of the invention.

[0026]FIG. 2 shows a cross-section through lines II-II in FIG. 1.

[0027]FIG. 3 shows a partial cross-section made through a heat exchanger in accordance with another embodiment of the invention.

[0028]FIG. 4 shows a heat exchanger with a plurality of liquid inlets and liquid outlets.

[0029]FIG. 5 is a cross-section through lines V-V in FIG. 4.

[0030]FIG. 6 is a schematic illustration of a garment in accordance with an embodiment of the invention applied to a patient.

[0031]FIG. 7 is a side elevation of the patient with the garment of the invention.

[0032]FIG. 8 is a top planar view of a garment in accordance with an embodiment of the invention.

[0033]FIG. 9 shows the top portion of the garment as applied over a patient's head.

[0034]FIG. 10 shows a top elevational view of a garment in accordance with an embodiment of the invention adapted for use for dogs.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0035] In accordance with the invention a heat exchanger in the form of a flexible sheet, which is highly effective in transferring a heat to or from a body, e.g. a living body, in order to control the body's temperature, is provided. The heat exchanger is made of a porous material having impermeable faces (to be referred to herein also as “skin”) as well as sealed edges to define within it a liquid-tight space.

[0036] The porous material is preferably highly hydrophilic, i.e. may absorb up to 25 times it's weight in liquid, e.g. water, and is highly elastic (capable of 20%-40% elongation with moderate tension). These features confer softness and pliability to the heat exchanger which permit it, in use, to closely follow the body's contours. Further, being made from a stretchable material minimizes the number of different sizes of heat exchanger garments typically required to be held in stocks.

[0037] The heat exchanger provided has an effective heat transfer, it is easy to handle and easy to stretch over the body and further it is soft and thus physiologically compatible with the skin and gives rise to less pressure sores as compared to prior art heat exchangers. These properties of the heat exchanger make it particularly useful for applications over sensitive skin, for example, for infants, subjects with wounded skin, e.g. with burn injuries, etc.

[0038] The use of a porous material to form the heat exchanger of the invention provides a product typically light in weight Tis feature, among others, eases the movement of the subject onto which the heat exchanger is applied, which is particularly advantageous during non-medical applications, some of which are described hereinafter.

[0039] According to one embodiment of the invention, the porous material is a polyvinyl alcohol (PVA) based foam known to posses the above detailed characteristics. An example of a PVA which may be used to form the heat exchanger of the invention is that manufactured by Hydrofera (Hydrofera, Conn., USA). The elasticity of this specific porous material was measured and was found to be of approximately 30% under moderate tension when wet.

[0040] More preferably, the heat exchanger of the invention is comprised of a core layer of PVA sandwiched between two external layers of silicon skins cured to the internal PVA layer. The heat exchanger according to this preferred embodiment may include additional an outer porous sheet, with a liquid impermeable interface therebetween, the additional external layer, which, upon use, is brought into contact with the subject's body, may function as an absorbent layer to absorb body fluids released from the subject's body, during the medical procedure, such as blood or sweat whilst the outwardly facing layer may function as an insulator to minimize heat transfer from the heat exchanger to the environment.

[0041] Preferably, the heat exchanger is bio-compatible (i.e. does not give rise to allergic reactions or skin irritations), durable (i.e. strong enough to withstand normal handling during its use), long lasting (does not decompose with time and, if desired, may be used more than once) and resistant to chemicals such as alcohol, ether, iodine, etc., typically used during the relevant medical procedures, e.g. in operation rooms.

[0042] The impermeable skin may be produced integrally with the open cell core (the porous material) and may be obtained by solvent welding or by sintering the outer surfaces of the porous material, e.g. by heat-induced sintering, to form closed cell films. In addition, the skin may also be produced by adhering a film of liquid impermeable material, e.g. a thin rubber sheet, onto the face of the porous sheet. Furthermore, such a skin may be formed by a skin-forming adhesive material such as a hot-melt glue, or liquid- or semi liquid rubber, cured in place. The skin, being typically a thin layer covering the faces of the open cell core, serves as a liquid barrier whilst retaining, to a large extent, the inherent elasticity and flexibility of the inner porous material. The impermeable layers also provide the heat exchanger with its resistance to accidental puncture due to their high elasticity.

[0043] The open cell core defined between the impermeable layers, constitutes the liquid transfer space and has typically a volume per unit area which is essentially the same throughout the entire garment. Preferably, such a core has a thickness of 1-3 mm and provides for a uniform flow of the liquid therethrough.

[0044] The sheet, typically formed into a garment for overlaying substantial potions of the subject's body, is designed for working under liquid pressure of about 0.20-0.5 atmosphere, which is less than the skills bursting pressure by a reasonable margin. Typically, the liquid transfer space has a volume per unit area of the sheet in the range of 0.75-2.5 liter/m² and preferably about 1.0 liter/m².

[0045] The porous sheet enables good liquid distribution within the liquid transfer space. The flow path of the liquid within the liquid transfer space may be defined by means of fluid-tight partitions formed within the liquid transfer space, if required.

[0046] The heat exchanger has typically a flow resistance such that at the working pressure it has a flow rate of 0.5-2.0 liter/min., preferably, about 1 liter/min. The liquid's circulation within the liquid transfer space is preferably obtained, but not limited thereto, by means of a pump.

[0047] The heat transfer liquid is typically water, the temperature of which preferably does not exceed 45° C.

[0048] The heat exchanger or garment is provided by at least one liquid inlet and at least one outlet tubes. For example, these tubes may have an internal diameter of 4-8 mm and an outer diameter of 6-10 mm, respectively.

[0049] According to another embodiment of the invention, the porous sheet may be laminated to additional layers of the porous sheet, which may have the same or different thickness as that of the liquid transfer layer. For example, in a triple-layer heat exchanger the liquid transfer space-forming layer is sandwiched between two additional porous sheets, with a liquid impermeable interface therebetween. In this particular case, the external layer, which, upon use, is brought into contact with the subject's body, may function as an absorbent layer to absorb body fluids released from the subject's body, during the medical procedure, such as blood or sweat, whilst the outwardly facing layer may function as an insulator to minimize heat transfer from the heat exchanger to the environment.

[0050] The heat exchanger having the aforementioned characteristics may be used for any application requiring a flexible heat exchanger with a relatively large heat exchange surface. A significant feature of the heat exchanger of the present invention is that it is of low cost, inter alia, since the flexible sheet, from which it is made, may be produced as a continuos web from which the individual heat exchangers may be cut; it is also of high efficiency and has an overall performance (heat exchange properties, flexibility, internal pressure resistance, corrosion resistance, conformability, resistant to blockage by inadvertent folding and others) hitherto attained only with more complex and costly devices. One advantage of having a low cost heat exchanger is that it may be disposed after a single use.

[0051] The garment of the invention is preferably designed such that each different section can be manipulated independently from other sections between an overlaying and a non-overlaying state. This has several advantages. For one, it allows easy accessibility to different parts of the body. Furthermore, this allows to use a single type of garment for different kinds of operations: for example in case of an operation performed on the subject's leg, the respective section will obviously be in the non-enveloping state while the other sections, namely those of the torso, the other leg, the arms, the neck, the head, etc., may be in an enveloping state. The feature of independent manipulation of the different sections between an enveloping and a non-enveloping state is typically achieved by emarginations in the garment's edges.

[0052] The garment may also have a modular design. For example, there may be a garment piece designed for the torso, a garment piece designed for the head, a garment piece designed for the legs or arms, etc. The different garment pieces may be combined to form a garment in a manner to meet specific needs.

[0053] The above feature provides an effective cover of the living body by the garment sufficient to properly control the body temperature, for a wide variety of different medical procedures. One should also note that effective heat transfer may be achieved regardless of the patient's position: whether he is placed lying on his back or whether he is placed with his back up or whether he is placed lying on his side, or even when positioned in a sitting position.

[0054] The heat exchanger of the invention is particularly useful in controlling the body temperature of a patient during surgery. Another important utility of the inventive heat exchanger is in control of body heat of infants, particularly prematurely born infants. The heat exchanger in accordance with the invention may be designed for all shapes and sizes of human subjects as well as for animals. Nevertheless, the heat exchanger of the invention may be used for other applications requiring body temperature control (heating and cooling) with a highly flexible, low weight system. This may include, inter alia, body temperature control of fireman, foundry workers, air crew personal, divers etc.

[0055] Notwithstanding the above, the heat exchanger of the invention may be applicable for controlling the temperature of non-living bodies, were its inherent properties of flexibility, disposability, high corrosion resistance and large area capability are advantageous. Such fields may include, inter alia, controlling the temperature of elements having irregular shapes, isolation of bodies, for example, isolation of chemicals in cases where the use of conventional metallic heat exchangers is problematic due to their high corrosion, etc.

[0056] Referring first to FIGS. 1 and 2, showing a heat exchanger 20, formed in this specific illustration as a rectangle sheet 22 having a porous core 24 overlaid by external skin layers 26 and 28 to form a three layer structure. Skins 26 and 28 and skin tight edges 30 define together a liquid transfer space.

[0057] The heat exchanger has a liquid inlet 32 and a liquid outlet 34. A liquid-tight partition 36 is formed within the sheet 22 to define a flow path between the liquid inlet 32 and the liquid outlet 34 as represented by arrows 38. Liquid-tight partition 36 may be formed by means of heat induced sintering of the open cells within the porous sheet by injection of a liquid or semi-liquid adhesive into the sheet, etc.

[0058] It should be noted that in other embodiments of the invention the liquid inlet and liquid outlet may be at different locations in the heat exchanger, without there being liquid-tight partitions to define a flow path. In such a case, the flow path is constituted by the entire sheet while the distribution of the liquid throughout the liquid transfer space is ensured by the porosity of said space and the consequent tortuous path of the liquid within said space.

[0059] The heat exchanger is provided with adhesive slips 40 which can adhere to flaps 42 at the opposite end of the sheet whereby the heat exchanger can be fixed, depending on its size, around the body, e.g. around an individual's torso, arm, leg, etc.

[0060] A preferred three layer structure according to the present invention is that consisting of an internal porous core made of PVA sandwiched by two external skin layers of silicon film, to form a heat exchanger consisting of silicon film-PVA-silicone film.

[0061] The ‘silicon film-PVA-silicone film’-containing heat exchanger of the invention is obtained by forming essentially mechanical bonds between the silicon and the PVA sheet. To this end a thin layer of premixed silicon is formed on one face of a PVA sheet under heat and pressure conditions (e.g. 100-120° C. and 1 psi). As a result, the silicon which is yet in a fluid state rapidly polymerized (20-30 seconds) while “interlocking” with the pores of the PVA. The silicon thus formed fuses together with the porous PVA in the form of a layer of an even thickness. The process may then be repeated to form a second thin silicon skin on the other face of the PVA sheet, thus forming the three layer flexible sheet.

[0062] The thickness of the two external silicon layers in the silicon/PVA/silicon sheet thus formed was about 0.1 mm. The internal layer may vary within the range of 1-3 mm. In contrast to most film forming operations on other porous materials (e.g polyurethane plastic), the resulting three layer sheet retains the elasticity and flexibility characteristics of the starting materials which allowing its sufficient stretching around a body to conform with the body's contours.

[0063] In addition, the three layer sheet showed to employ substantially all (nearly 100%) of its transfer space for heat transfer, as opposed to other heat exchanges having much less active transfer space, (typically 50%-60%) due to, for example, the existence of welds within the sheet or other support structures.

[0064] The interfacial resistance to heat transfer of the above heat exchanger was also evaluated. The results showed that the heat exchanger has resistance of 0.01 C/W/m² as compared to values between 0.025 to 0.05 C/W/m² obtained with other commercially available heat exchangers. These results indicate that the silicon/PVA/silicon containing heat exchanger of the present invention is capable of transferring the same amount of heat, however, using only 20%-40% of the transfer space that other heat exchangers require.

[0065] The heat exchanger may contain a fourth layer made of a porous material (PVA cotton fabric, etc.) attached on one or both of the skins of the heat exchanger and employed as absorbing layers for body fluids including blood or sweat and others fluids surrounding the heat controlled body as well as for preventing pressure sores (due to the additional layer's softness).

[0066] Another embodiment of the heat exchanger is represented by FIG. 3. In this embodiment the heat exchanger, which is represented in a cross-section only, is a three layer structure consisting of two external porous sheets 50 and an internal porous sheet 52, attached to one another with a impermeable interface 54 between each external layer 50 and the internal layer 52. The internal layer constitutes the liquid transfer space, which is defined between the liquid impermeable interfaces 54, and in which the heat transfer liquid flows as represented by arrows 56. The inwardly, skin-facing external layer may serve for absorption of body fluids including blood or sweat and others, while the outwardly facing external layer may serve to improve the thermal isolation of the heat exchanger.

[0067] A heat exchanger according to another embodiment of the invention is shown in FIGS. 4 and 5. This heat exchanger has an internal structure which may be the same as that illustrated in FIGS. 2 and 3. The heat exchanger 60 is here again made of a rectangular sheet 62 and is connected to a liquid inlet 64 and a liquid outlet 65. The liquid is introduced into the liquid transfer space via a plurality of lateral openings 66 and 67, respectively, within a feed manifold 64′ and a drain manifold 65′, to allow an essentially uniform liquid flow through the liquid transfer space, represented by arrows 68. The design wherein the feeding of the liquid is from one edge of the heat exchanger while from the opposite edge the liquid is drained provides the system with good liquid distribution in which a liquid front may be formed (i.e. liquid enters and exists the liquid transfer space at the same time) and low pressure drop.

[0068] A garment 70 in accordance with an embodiment of the invention, fitted over an individual 72 can be seen in FIG. 6. In this case, the liquid inlet and outlet 74, 76, respectively, are fitted in the garment in a shoulder section 78. In addition to shoulder section 78, the garment consists of a combined torso-thigh section 80. The fastening of the garment in an enveloping state is achieved by means of straps 82, particularly hook and pile-type fastening straps.

[0069] A side view of the patient with a garment of the embodiment of FIG. 6 can be seen in FIG. 7, with the patient placed on an operating table 84. Inlet and outlet pipes 74, 76 are connected to a heat transfer liquid control and circulation device 90. The fluid is typically a liquid such as water.

[0070] Reference is now being made to FIG. 8, showing a garment 100 in accordance with another embodiment of the invention. The garment is symmetrical along a longitudinal midline, which is a typical, but not exclusive case. Occasionally, garments for a variety of uses may also be made to be non symmetrical. As can be seen, the garment has a variety of dedicated sections a, b, c, d and e on both the left and right sides of the garment. The different sections are defined by the general shape of the garment as well as by emarginations 150, 152, 154, 156 and 158, part of which are in the form of lateral indentations while others are in the form of slits. Section a is intended for enveloping portions of the individual's neck and head, section b is for portions of the individual's arms, section c is for side portions of the individual's torso, section d is for the hip and thigh while section e is for the skin.

[0071] The garment is provided with a fluid inlet 160 and a fluid outlet 162, which in this specific embodiment are fitted into section a. As will no doubt be appreciated, this is a mere example and the outlet and inlet may be fitted into any other section, at times conveniently into section d or e at times conveniently into section c, etc. In addition, it is possible also to form the garment such that the fluid inlet will be in one section and the fluid outlet in another section of the garment.

[0072] In the garment, a flow path represented by a series of consecutive arrowed lines 166, is defined by means of flow path-defining liquid-tight lines, which are designated collectively by the numeral 168. These lines may be formed by a variety of means some of which have been mentioned above.

[0073] The garment 100 may be provided with integral fastening means, e.g. hook and pile-type fasteners or adhesive or pressure-sensitive patches. Alternatively, the fastening means may be applied prior to fitting the garment on the patient: for example, a two-sided patch may then be attached to the internal layer of the garment; separate straps or buckles, etc. may be added. All such fastening means are generally known per se.

[0074] Section a is of the garment 100 may be fitted over substantially all non-facial parts of the head as illustrated in FIG. 9.

[0075] The fixing of this garment is achieved, in this specific embodiment, by attachment patches 180 at edges of the garment. These attachment patches, typically an extension of the garment's external layer, may be covered by an adhesive material suitable for adherence to the skin or to an overlapping portion of the garment, or may be fitted with one attachment member of the hook and pile-type attachment couple, (e.g. VELCRO#-type attachment). Alternatively, the fixing of the garment may be achieved by using a sticky adhesive over the whole or part of the inside surface of the garment for adhering directly onto the skin.

[0076] The garment in accordance with another embodiment of the invention is shown in FIG. 10. Garment 200 of this embodiment is designed for veterinary use, e.g. for a cat or a dog. Different elements of the garment of FIG. 10 being functional equivalents to those of the garment 100 of FIG. 8, have been given like reference numerals shifted by 100, while the elements marked by letters were marked by corresponding letters with a 2 index. The reader is referred to the description of FIG. 8 for proper explanation of the function of such elements.

[0077] The garment 200 of FIG. 10 is fitted with adhesive patches 282 whereby the garment can be fastened in the enveloping state to the animal's body, or to overlapping segments of the garment. The garment has a section f which may be forward folded over a lower abdominal part of the and which is formed with a flap 204 which once folded forms an opening for the subject's tail.

[0078] As will be appreciated, the illustrated embodiments are but examples. The garment may be formed at a myriad of different shapes and sizes for different medical procedures and subjects. 

1. A heat exchanger comprising: a flexible sheet comprising a porous material sandwiched between two liquid impermeable layers, the liquid impermeable layers defining between them and between fluid-tight edges, a liquid transfer space within said sheet; and at least one liquid inlet and at least one liquid outlet for respective connection to a source and a drain of a heat transfer liquid, with a flow path defined between the liquid inlet and liquid outlet within said liquid transfer space.
 2. A heat exchanger according to claim 1, wherein said porous material has a porosity of about 30-95%.
 3. A heat exchanger according to claim 1, wherein said sheet has an elasticity characterized in about 20-40% elongation with moderate tension.
 4. A heat exchanger according claim 1, wherein said porous material is a PVA foam.
 5. A heat exchanger according to claim 1, wherein said liquid transfer space has a thickness of about 1-3 mm.
 6. A heat exchanger according to claim 1, wherein said liquid impermeable layers are formed by sintering faces of said porous material.
 7. A heat exchanger according to claim 6, wherein said sintering results from heating.
 8. A heat exchanger according to claim 1, wherein said impermeable layers are formed from an adherent, film forming material.
 9. A heat exchanger according to claim 8, wherein said material is a rubber-based hot melt glue.
 10. A heat exchanger according to claim 8, wherein said material is a liquid or semi-liquid rubber cured in place.
 11. A heat exchanger according to claim 1, wherein said liquid impermeable layers are formed by a liquid impermeable film adhered to the porous material.
 12. A heat exchanger according to claim 1, comprising at least two sheets of porous materials attached to one another with an impermeable interface between them, said liquid transfer space is formed within one of said sheets sandwiched with an additional impermeable layer.
 13. A heat exchanger according to claim 12, comprising two external layers of porous material sandwiching an internal layer of porous material, there being liquid impermeable interface between each of the external layers and the internal layer, the two liquid impermeable interfaces defining between them said liquid transfer space within the internal layer.
 14. A heat exchanger according to claim 13, formed by adhering three sheets of porous material to one another.
 15. A heat exchanger according claim 1, wherein said flow path is defined by means of fluid-tight partitions formed within the liquid transfer space.
 16. A heat exchanger according claim 1, wherein said at least one liquid inlet and said at least one liquid outlet comprise a respective feed and drain manifold.
 17. A heat exchanger according to claim 16, wherein the manifolds comprise a plurality of openings in a liquid duct, communicating with said liquid transfer space.
 18. A heat exchanger according to claim 17, wherein said duct is integrally formed with said flexible sheet.
 19. A heat exchanger according to claim 1, for controlling a subject's body temperature, the heat exchanger formed into a garment for overlaying substantial portions of said body.
 20. A heat exchanger comprising: a flexible sheet of polyvinyl alcohol (PVA) sandwiched between two liquid impermeable layers, the liquid impermeable layers defining between them and between fluid-tight edges, a liquid transfer space within said sheet; and at least one liquid inlet and at least one liquid outlet for respective connection to a source and a drain of a heat transfer liquid, with a flow path defined between the liquid inlet and liquid outlet within said liquid transfer space; said flexible sheet of the heat exchanger being adapted for overlying portions of a body.
 21. A heat exchanger according to claim 20, wherein said liquid transfer space has a thickness of about 1-3 mm.
 22. A heat exchanger according to claim 20, wherein said impermeable layers are formed from an adherent, film forming material.
 23. A heat exchanger according to claim 22, wherein said material is a liquid or semi-liquid rubber cured in place.
 24. A heat exchanger according to claim 22, comprising at least two sheets of porous materials attached to one another with an impermeable interface between them, said liquid transfer space is formed within one of said sheets sandwiched with an additional impermeable layer.
 25. A heat exchanger according to claim 24, comprising two external layers of porous material sandwiching an internal layer of porous material, there being liquid impermeable interface between each of the external layers and the internal layer, the two liquid impermeable interfaces defining between them said liquid transfer space within the internal layer.
 26. A heat exchanger according to claim 20, wherein said at least one liquid inlet and said at least one liquid outlet comprise a respective feed and drain manifold.
 27. A heat exchanger according to claim 26, wherein the manifolds comprise a plurality of openings in a liquid duct, communicating with said liquid transfer space.
 28. A heat exchanger according to claim 27, wherein said duct is integrally formed with said flexible sheet.
 29. A heat exchanger comprising: a flexible sheet of polyvinyl alcohol (PVA) sandwiched between two silicon impermeable layers, the silicon impermeable layers defining between them and between fluid-tight edges, a liquid transfer space within said sheet; and at least one liquid inlet and at least one liquid outlet for respective connection to a source and a drain of a heat transfer liquid, with a flow path defined between the liquid inlet and liquid outlet within said liquid transfer space; said flexible sheet of the heat exchanger being adapted for overlying portions of a body.
 30. A heat exchanger according to claim 20 or 29, for controlling a subject's body temperature, the heat exchanger formed into a garment for overlaying substantial portions of said body. 